There is currently a growing need for high-speed and high-capacity communications. To meet such a need, a communication system of wavelength division multiplex (WDM) for transmitting multiplexed light rays is used. Available as one of the WDM communication systems is an optical ring network system including an optical ring network having optical transmission paths connected in a ring configuration. Even if a fault occurs in a part of the optical transmission paths, communications are maintained by switching from one communication line to another.
Several schemes of switching communication lines responding to an optical transmission path malfunction in the optical ring network have been proposed. For example, Japanese Laid-open Patent Publication No. 6-97950 discloses a first communication line switching scheme. In the first communication line switching scheme disclosed, each optical transmission apparatus connected to an optical ring network causes the same information to be carried on a first-wavelength optical signal and a second-wavelength optical signal. The optical transmission apparatus transmits the first-wavelength optical signal and the second-wavelength optical signal in mutually opposite directions over the optical ring network. During normal operation, the optical transmission apparatus selectively receives the first-wavelength signal. In the case of a malfunction, if the optical transmission apparatus does not receive the first-wavelength signal, the apparatus selectively receives the second-wavelength signal.
Japanese Laid-open Patent Publication No. 6-97950 also discloses a second switching scheme. In accordance with the second switching scheme disclosed, the optical transmission apparatus transmits information on the first-wavelength optical signal during normal operation. In the case of a malfunction, an optical transmission apparatus having detected the fault occurring in the optical transmission path adjacent thereto converts the optical signal from the first wavelength into the second wavelength. The optical transmission apparatus having detected the fault transmits the second-wavelength optical signal into the optical transmission path in the direction opposite to the incoming direction of the first-wavelength optical signal.
In accordance with a third switching scheme disclosed in Japanese Laid-open Patent Publication No. 2000-332805, an optical ring network is arranged for each wavelength. At each node, the optical signal of each wavelength is input to an allocated optical ring network. Upon detecting a malfunction in any optical ring network of the working wavelength, an optical ring apparatus connected to each node switches from the optical ring network currently transmitting the optical signal to the optical ring network for transmitting another wavelength.
In accordance with the first switching scheme, two-way channels on a single wavelength are always used in the optical ring network in order to transmit a single piece of information. The number of lines available on the optical ring network is limited to the number equal to or smaller than the number of wavelengths available on the optical ring network. In accordance with the second switching scheme, the optical transmission apparatus adjacent to the optical transmission line with a fault detects the fault and then loops back the optical signal. The optical signal thus is transferred twice along an optical communication path between the optical transmission apparatus having transmitted the optical signal to the optical ring network and the optical transmission apparatus having detected the fault. If a large number of optical transmission apparatuses are present over the communication path, or if the distance between the apparatuses is long, a transmission delay caused in a bypass communication line bypassing the fault increases. The optical ring network of the third switching scheme includes a backup optical transmission line that is not used during normal operation when the network is not malfunctioning. Each optical ring apparatus forming the network is provided with a wavelength switching function and a branching function corresponding to all wavelengths used in the network system. The cost of the optical ring network system is thus high.